import java.util.*;

public class Leetcode {
}

//leetcode:LCR 114.火星词典
class Solution {
    Map<Character, Set<Character>> edges = new HashMap<>();
    Map<Character,Integer> in = new HashMap<>();
    boolean check;
    public String alienOrder(String[] words) {
        for(String s : words){
            for(int i = 0; i < s.length(); i++){
                char ch = s.charAt(i);
                in.put(ch,0);
            }
        }

        //建图
        int n = words.length;
        for(int i = 0; i < n;i++){
            for(int j = i+1; j < n; j++ ){
                add(words[i],words[i+1]);
                if(check == true) return "";
            }
        }

        Queue<Character> q = new LinkedList<>();
        for(char ch : in.keySet()){
            if(in.get(ch) == 0) q.add(ch);
        }

        StringBuffer ret = new StringBuffer();
        while(!q.isEmpty()){
            char t = q.poll();
            ret.append(t);

            if(!edges.containsKey(t)) continue;

            for(char ch : edges.get(t)){
                in.put(ch,in.get(ch)-1);
                if(in.get(ch) == 0) q.add(ch);
            }
        }

        //判断
        for(char ch : in.keySet()){
            if(in.get(ch) != 0) return "";
        }
        return ret.toString();
    }

    public void add(String s1, String s2){
        int n = Math.min(s1.length(),s2.length());
        int i = 0;
        for(; i < n;i++){
            char c1 = s1.charAt(i);
            char c2 = s2.charAt(i);
            if(c1 != c2){
                if(!edges.containsKey(c1)){
                    edges.put(c1,new HashSet<>());
                }
                if(!edges.get(c1).contains(c2)){
                    edges.get(c1).add(c2);
                    in.put(c2,in.get(c2)+1);
                }
                break;
            }
        }
        if(i == s2.length() && i < s1.length()) check = true;

    }
}
